1. Field of Invention
The present invention relates generally to the detection of leakage signals and the location of leakage sources, and more particularly to methods and apparatus for detecting digital signals leaking from the coaxial cable portion of an HFC network and for locating the source or sources of the leaks.
2. Background Art
The tasks of detecting signals leaking from the coaxial cable portion of a hybrid fiber-coax (HFC) network and locating the source or sources of the leak are very important for controlling ingress and egress in an HFC network, such as an HFC cable television network.
The known methods of leakage detection and location in HFC cable television networks can be divided into two groups. The first group involves the use of analog TV signals. The second group involves methods in which special pilot signals or carriers are inserted into the forward path spectrum and then these signals are used to detect leakage. Both groups may also include some form of tag signal or modulation imposed on the analog TV signal or pilot signal, to mark the received signal as leakage originating from the HFC network.
Examples of the first group are found in the following patent documents: Pub. App. No. 2008/0133308 (Jun. 5, 2008) to Harris; Pub. App. No. 2008/0033698 (Feb. 7, 2008) to Stelle; Pub. App. No. 2007/0022457 (Jan. 25, 2007) to Eckenroth et al.; U.S. Pat. No. 7,548,201 to Eckenroth et al.; U.S. Pat. No. 7,395,548 to Runzo; U.S. Pat. No. 6,801,162 to Eckenroth et al.; Pub. App. No. 2006/0248565 (Nov. 2, 2006) to Shimp et al.; U.S. Pat. No. 6,833,859 to Schneider et al.; U.S. Pat. No. 6,313,874 to Bowyer et al.; and U.S. Pat. No. 5,777,662 to Zimmerman. Examples of the second group are found in the following patents: U.S. Pat. No. 6,600,515 to Bowyer et al.; and U.S. Pat. No. 4,072,899 to Shimp. The use of tag signals in connection with analog TV signals or pilots signals are disclosed in the following patent documents: U.S. Pat. No. 6,804,826 to Bush et al.; U.S. Pat. No. 6,600,515 to Bowyer et al.; and U.S. Pat. No. 6,018,358 to Bush.
The known methods have limitations in cases where the HFC network transmits only digital signals, such as digital TV signals, like quadrature amplitude modulation (QAM) signals (“all-digital system”). Many modern all-digital systems do not have analog channels (or carriers) and do not have empty bandwidth for test signals or pilot carriers. Current leakage detection techniques are not effective on leaks from QAM TV signals or other digital TV signals, because the leakage signal looks like pure noise. Using a QAM receiver and demodulator also does not help to detect leakage from a QAM signal, because QAM-64 demodulators, e.g., need a carrier to noise ratio (CNR) of better than 20 dB. Thus, the only practical approach using known techniques in an all-digital system is to allocate expensive bandwidth for an analog test or pilot signal to be detected as a leakage signal.
The problem of detecting leakage in all-digital networks is well described in the following articles by Ron Hranac: “Broadband: Signal leakage in all-digital network” http://www.cable360.net/ct/operations/bestpractices/33882.html (Feb. 1, 2009) and “Broadband: Signal leakage in all-digital network: Continuing story” http://www.cable360.net/ct/sections/-columns/broadband/35443.html (May 1, 2009). Also this problem is well discussed in the publication, “Leakage in all-digital World” http://www.cablefax.com/technology/strategy/-Leakage-in-an-All-Digital-World—34303.html (Mar. 1, 2009).
There are known systems and methods for locating leakage sources based on triangulation, using directional antennas. Examples of known systems and methods using triangulation are disclosed in the following patent documents: Pub. App. No. 2008/0133308 (Jun. 5, 2008) to Harris; Pub. App. No. 2007/0022457 (Jan. 25, 2007) to Eckenroth et al.; U.S. Pat. No. 7,548,201 to Eckenroth et al.; and U.S. Pat. No. 6,801,162 to Eckenroth et al. Also, some known methods are based on obtaining GPS coordinates of a roaming vehicle using a non-directional leakage detector antenna and measuring the strength of the leakage signal. Such methods are based on making many measurements of leakage signal strength, from many different points, to achieve sufficient accuracy for locating the leakage source. As a result, such methods require relatively long periods of time to locate leakage sources. Examples of such methods are disclosed in the following patent documents: Pub. App. No. 2008/0133308 (Jun. 5, 2008) to Harris; and U.S. Pat. No. 7,360,124 to Bouchard.